Turbine or compressor blades and turbine or compressor rotors are components subjected to both high thermal and mechanical loading. To reduce the thermal loading, which the materials used, in particular chrome steels or nickel-based alloys or the like, are exposed to during the operation of the turbine or of the compressor, such components are normally provided with internal cooling passages. A mostly gaseous or vaporous cooling medium, such as cooling air for example, flows through the cooling passages during operation, in the course of which mainly convective cooling is effected by heat transfer from the wall regions defining the respective cooling passage to the cooling medium flowing past. In order to achieve as uniform a cooling as possible of all the relevant regions of the component, e.g. a turbine blade, a meander-shaped course of the cooling passages or cooling air passages inside the component, in particular in the airfoils of turbine blades, is provided as a rule. On account of the restricted spatial conditions inside the airfoil, comparatively small cross sections and comparatively small radii of curvature are partly necessary.
Often used is an “open” cooling concept in which the cooling medium, after flowing through the respective cooling passage, leaves the component to be cooled via outlet passages branching off from the cooling passage and opening into outlet openings at the surface in order to be subsequently mixed with the hot working or flow medium flowing through the flow passage of the turbine or of the compressor. The outlet openings may be designed and arranged in particular like “film-cooling openings”, such that the cooling medium flowing off from them flows along the surface of the component and in the process forms a cooling film protecting the surface material from direct contact with the hot and corrosive working medium.
Despite such polished and constantly refined cooling concepts, the thermal loading of turbine blades of gas or steam turbines is considerable. There is also the mechanical loading on account of the centrifugal forces which occur, in particular at the moving blades arranged on the turbine shaft and rotating at a high speed; but mechanical stresses on account of vibrations or impacts, etc., also often lead to pronounced loading. In particular during repeatedly occurring load alternation actions and in start-up and shutdown situations, in conjunction with variations in the speed of rotation, material fatigue phenomena occur during continued operation of the turbine or of the compressor despite novel materials optimized with respect to fatigue strength. Such fatigue phenomena in the form of microscopic cracks, etc., limit the period of use or the service life of the respective component.
A turbine blade described above and cooled in an open circuit is known, for example, from US 2003/143075 A1. To cool their trailing edge by blowing out turbulated cooling air, the turbine blades are provided with especially small blow-out holes which have been produced by means of a special method. This method provides for a mandrel contoured along its extent to be inserted into a hole provided in the trailing edge. The material of the trailing edge surrounding the holes is then plastically deformed by pressing together the outer walls of the trailing edge in such a way that contoured blow-out holes provided with turbulators remain behind after the removal of the mandrel. According to US 2003/143075 A1, care is to be taken here to ensure that the overall deformation of the turbine blade is minimal in order to keep the stress within its material as low as possible.
In addition, an autofrettage process for introducing residual compressive stresses into a pipe of a common-rail injection system is known from US2005/005910 A1.
On the whole, therefore, in the interest of operating reliability, comparatively frequent inspection and possibly exchange or renewal of the component are necessary, which involves undesirable downtimes and high costs. Since the service life of the turbine or compressor component of interest here can generally be estimated only with difficulty a priori, inspections carried out according to schedule, with service intervals estimated rather on the conservative side, i.e. service intervals selected to be rather short, often prove later to be unnecessary, since the material fatigue at the time of inspection has still not advanced as far as feared.